Relative-signal-level data detection from a spread-spectrum signal using matched-filter obtained side information

ABSTRACT

An improvement to a spread-spectrum base station receiver having a matched filter. A symbol sampler samples at the symbol time T S , a plurality of symbol samples from the matched filter. A relative-signal-level decoder decodes the plurality of received-symbol samples, thereby generating a plurality of decoded-symbol samples. A noise sampler samples at a plurality of chip times kT C , but not at the symbol time T S , a plurality of noise samples from the matched filter, before, after, or a combination of before and after, a symbol sample. An estimator processes the plurality of noise samples. The erasure detector detects for each decoded-symbol sample from the plurality of decoded-symbol samples and from the plurality of noise samples, an erasure condition for the corresponding decoded-symbol sample, and thereby generates an erasure signal. An erasure decoder erasure decodes the input data using the erasure signals from the erasure detector.

BACKGROUND OF THE INVENTION

This invention relates to spread-spectrum communications in acode-division-multiple-access system, and more particularly to usingside information from noise samples at an output from a matched filterat times other than a sampling time for a symbol sample, to aid inforward error correction decoding.

DESCRIPTION OF THE RELEVANT ART

In a direct-sequence (DS) code-division-multiple-access (CDMA) systemhaving a base station and a plurality of remote stations transmitting tothe base station, the spread-spectrum signals from many of the remotestations arrive at the base station simultaneously. The spread-spectrumsignal from each remote station may arrive at the base station with adifferent power level with different symbol and chip arrival times.Further, the desired spread-spectrum signal at a particularspread-spectrum receiver receiving a particular spread-spectrum channelfrom a particular remote station, may be fading, and is, on occasion,not detectable, or has a high error rate.

Diversity coding, forward-error-correction (FEC) decoding, andinterference cancellation are approaches to reducing the error rates.RAKE may be used to combine the strongest signal paths in a fading ormultipath environment. These approaches do not, in general, takeadvantage of the unique noise environment of a DS-CDMA system, in whichnoise, on the average, is due to the multiple spread-spectrum signalsfrom the plurality of remote stations.

SUMMARY OF THE INVENTION

A general object of the invention is to reduce error rate in adirect-sequence code-division-multiple-access (DS-CDMA) spread-spectrumsystem.

Another object of the invention is to use the noise interference fromthe multiple users in the DS-CDMA system as side information in reducingerror rate for decoding differentially-encoded data.

According to the present invention, as embodied and broadly describedherein, an improvement to a spread-spectrum receiver at the base stationin a direct-sequence code-division-multiple-access (DS-CDMA) system isprovided. The spread-spectrum receiver, in a DS-CDMA system has, at aninput, a plurality of spread-spectrum signals, arriving from a pluralityof remote users, respectively. Each spread-spectrum signal in theplurality of spread-spectrum signals has a differentially encoded-datasymbol. Each differentially encoded-data symbol is spread-spectrumprocessed by a chip-sequence signal lasting a symbol time T_(S). Eachremote user may be operating at a different symbol time T_(si), where iis an index for the different symbol time. Each chip-sequence signal inthe plurality of chip-sequence signals is different, due to a differentchip sequence, from other chip-sequence signals used by otherspread-spectrum signals in the plurality of spread-spectrum signals.

Each spread-spectrum receiver in the base station includes a matchedfilter having an impulse response matched to a desired chip-sequencesignal in the plurality of chip-sequence signals. The matched filterdetects a desired spread-spectrum signal in the plurality ofspread-spectrum signals arriving at the spread-spectrum receiver at thebase station. The desired spread-spectrum signal is spread-spectrumprocessed with a desired chip-sequence signal.

The improvement comprises a symbol sampler, a noise sampler, arelative-signal-level decoder, an estimator, an erasure detector, and anerasure decoder. The symbol sampler samples at a plurality of symboltimes nT_(S), a plurality of symbol samples from the desired matchedfilter. The integer n indexes the plurality of symbol times. Each symbolsample has time duration T_(S). The relative-signal-level decoderdecodes, with reference to the relative-signal-level of the current andpreviously received symbol samples, the plurality of symbol samples,thereby generating a plurality of decoded-symbol samples. As a result ofnoise and interference, these samples are non-binary. Hard limitingthese samples prior to processing is not a preferred embodiment, but isan option included herein.

The noise sampler samples before, after, or a combination of before andafter each symbol sample at a plurality of chip times kT_(C), but not atthe symbol time T_(S), a plurality of noise samples. The estimatorprocesses the plurality of noise samples to generate a noise estimate.The erasure detector detects, for each symbol sample and from the noiseestimate, an erasure condition, and thereby generates an erasure signal.In response to the data and the erasure signals, the erasure FECdecoder, erasure decodes the symbols, as is well known in the art.

Additional objects and advantages of the invention are set forth in partin the description which follows, and in part are obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention also may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 is a block diagram of a relative-signal-level data symboldetector using matched-filter obtained side information;

FIG. 2 shows sampling at chip time T_(c) and symbol time T_(S);

FIG. 3 shows average noise power during a symbol time T_(S);

FIG. 4 shows a threshold between a 1 and 0 bit; and

FIG. 5 shows an erasure region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference now is made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings.

The present invention provides an improvement to a spread-spectrumreceiver in a direct-sequence code-division-multiple-access (DS-CDMA)system. The DS-CDMA system is assumed to have a base station and aplurality of remote stations. At the base station a plurality ofspread-spectrum signals arrive from the plurality of remote stations,respectively. Each spread-spectrum signal modulates differentiallyencoded symbols. The differentially encoded symbols typically are datato be transmitted over a particular spread-spectrum channel. Moreparticularly, the input data are differentially encoded, usingtechniques well-known in the art.

The present invention is anticipated to be implemented with a digitalsignal processor (DSP) or application specific integrated circuit(ASIC). The means plus functions, and their embodiment as a “device”,“detector”, “decoder” and/or “estimator”, include the digital signalprocessor or application specific integrated circuit with software.Thus, a device, detector, decoder, and/or estimator, may be a part or aportion of a digital signal processor or ASIC, and software.

Each spread-spectrum signal in the plurality of spread-spectrum signalshas a chip-sequence signal lasting a symbol time T_(S). Each remote usermay be operating at a different symbol time T_(si), where i is an indexfor the different symbol time. Each chip-sequence signal in theplurality of chip-sequence signals is different from other chip-sequencesignals used by other spread-spectrum signals in the plurality ofspread-spectrum signals. Each chip-sequence signal is different since adifferent chip sequence is used for each chip-sequence signal in theplurality of chip-sequence signals.

The invention anticipates the use of interfering spread-spectrum signalsfrom the DS-CDMA system as side information to reduce error rate.Consider a received plurality of spread-spectrum signals r(t), whichincludes a desired spread-spectrum signal s_(o)(t) and a multiplicity ofinterfering spread-spectrum signals s_(i)(t), where i is an indexreferring to each of the multiplicity of interfering signals. Then thereceived plurality of spread-spectrum signals r(t) may be expressed as asum of the desired spread-spectrum signal s_(o)(t) plus the sum of theinterfering spread-spectrum signals s_(i)(t):

r(t)=s _(o)(t)+ΣS_(i)(t)

On the average, only half of the interfering spread-spectrum signals arechanging a data bit, or data symbol, from a +1 to a −1 symbol or bit, orfrom a −1 to a +1 symbol or bit, at any point in time. Thus, during asymbol time, 0<t≦T_(S), the interfering spread-spectrum signals s₁(t),s₂(t), s₃(t), . . . s_(N)(t), on average are a “one” (“1”) bit half ofthe time. Thus, half of the interfering spread-spectrum signals changefrom −1 to +1 and +1 to −1, and the other half of the interferingspread-spectrum signals do not change state, and go from +1 to +1 and −1to −1.

When the DS-CDMA system is operating at or near capacity, then thesignal-to-interference ratio (SIR) at the output of the matched filterof the spread-spectrum receiver may be 3 dB. With the observation, forthe DS-CDMA system, that if half of the interfering signals changedstate from +1 to −1 or vice versa, then the interference level does notchange for the other half of the interfering spread-spectrum signals.This is because half of the interfering spread-spectrum signalstransition from a −1 to a −1 or from a +1 to a +1, which result in nochange in signal level. Thus, on the average, half of the interferingspread-spectrum signals did not change state, then on the average, halfof the interfering spread-spectrum signals are constant. Thus, half ofthe noise, caused by the multiuser interference, is correlated, and halfof the noise is not correlated. This observation from the DS-CDMA systemis used to reduce error rate with the present invention.

In the exemplary arrangement shown in FIG. 1, the spread-spectrumreceiver includes despreading means, which may be embodied as a matchedfilter 21. The matched filter 21 has an impulse response matched to adesired chip-sequence signal in the plurality of chip-sequence signals.The matched filter 21 detects a desired spread-spectrum signal in theplurality of spread-spectrum signals arriving at the spread-spectrumreceiver. In general, the matched filter 21 is for a complex signal,that is, signals having an in-phase component and a quadrature-phasecomponent. Designing a particular embodiment for a complex signal isanticipated by the present invention. References to the various signals,symbols and estimate, in this disclosure includes embodiments as acomplex signal, to embodiments as a real signal, i.e., a real componentof a complex signal, and to embodiments as a magnitude of a complexsignal.

While the matched filter is the preferred embodiment, the despreadingmeans may be embodied as a correlator or a bank of correlators. Thecorrelator(s) would include a chip-sequence generator, for generating achip-sequence signal matched to the desired chip-sequence signal in theplurality of chip-sequence signals, as is well-known in the art.

Each spread-spectrum signal in the plurality of spread-spectrum signalshas a chip-sequence signal lasting a symbol time T_(S). Each remote usermay be operating at a different symbol time T_(si), where i is an indexfor the different symbol time. Each chip-sequence signal in theplurality of chip-sequence signals is different from other chip-sequencesignals used by other spread-spectrum signals in the plurality ofspread-spectrum signals. Each chip-sequence signal is different since adifferent chip sequence is used for each chip-sequence signal in theplurality of chip-sequence signals.

In the exemplary arrangement shown in FIG. 1, the spread-spectrumreceiver includes a matched filter 21, which has an impulse responsematched to a desired chip-sequence signal in the plurality ofchip-sequence signals. The desired chip-sequence signal is for thedesired spread-spectrum signal to be received by the receiver. Thematched filter 21 detects the desired spread-spectrum signal from theplurality of spread-spectrum signals arriving at the spread-spectrumreceiver.

The improvement comprises sampler means, relative-signal-level means,estimate means, erasure-detection means, and an erasure decoder 28. Thesampler means is coupled to the matched filter 21. Therelative-signal-level means is coupled to the sampler means. Theestimate means is coupled to the sampler means. The erasure-detectionmeans is coupled to the estimate means and to the sampler means. Theerasure decoder 28 has an erasure input coupled to the erasure-detectionmeans and a data input coupled to the relative-signal-level means.

The sampler means samples, as shown in FIG. 2, at a plurality of symboltimes nT_(S), the plurality of symbol samples from the matched filter21. The plurality of symbol times nT_(S) is the time occurrence of aplurality of symbol samples, and repeats every symbol time T_(S). Theinteger n is an index to each symbol time.

The sampler means samples at a plurality of chip times kT_(C) l but notat a plurality of symbol times nT_(S) , a plurality of noise samples,from the matched filter 21. The chip time T_(C) is the time duration ofa chip, and repeats every chip time T_(C). The sequence of chip times isindexed by factor k. The sampling of the plurality of noise samples mayoccur before, after, or a combination of before and after, the samplingat each symbol time for each symbol sample. FIG. 3 shows that the symbolsample for a particular sequence of symbols may be non-synchronous forsymbol samples for other sequences of symbol samples, from otherspread-spectrum channels.

The relative-signal-level means decodes adjacent symbol samples of theplurality of symbol samples, thereby generating a plurality ofdecoded-symbol samples. The decoding preferably is from subtracting thesignal level of adjacent symbol samples. The result is preferably anon-binary word, although hard limiting, which produces a binary word,could be used in a poorer quality system in which cost is of primaryconcern.

The estimate means estimates, or filters, a plurality of noise samplesfrom the sampler means, to generate a noise estimate. The noise estimatemay be a low-pass filtered version of the plurality of noise samples.Alternatively, using a digital signal processor embodiment orapplication specific integrated circuit (ASIC) embodiment, the estimatemeans may use a mathematical algorithm for estimating the level ofnoise. The mathematical algorithm may include, but is not limited to,straight averaging; root means square (RMS) averaging; and determining amedian value in the plurality of noise samples.

The erasure-detection means detects from the noise estimatecorresponding to a particular decoded-symbol sample from the pluralityof decoded-symbol samples, an erasure condition, and thereby generatesan erasure signal. The erasure condition might occur when the ratio ofthe particular decoded-symbol sample to the noise estimate, an SIR, isbelow a threshold, or when the magnitude of the difference between thedecoded-symbol sample corresponding to the noise estimate is below thethreshold.

The erasure decoder 28 may be embodied as an FEC decoder, and has anerasure input and a data input. The erasure input is coupled to theerasure-detection means, and the data input is coupled to therelative-signal-level means. The erasure decoder 28 erasure decodes eachdecoded-symbol sample, using a corresponding erasure signal. Typically,if the erasure signal from the erasure-detection means indicated a highprobability of error, that is, the signal level falls between levels Δ₁and Δ₂ in FIG. 5, then the erasure decoder 28 employs this addedinformation when processing the syndrome formed in the FEC decoder. FECerasure decoders are well known in the art and can be purchasedcommercially.

As illustratively shown in FIG. 1, the sampler means may include a noisesampler 22 and symbol sampler 23. The symbol sampler 23 is coupled tothe matched filter 21. The symbol sampler 23 samples at a plurality ofsymbol times nT_(S), a plurality of symbol samples. In a typicalembodiment employing a digital signal processor or an applicationspecific integrated circuit (ASIC), the symbol sampler 23 might be agate, for gating the symbol sample from the matched filter 21. Thetiming for sampling with the gate comes from timing circuit 33.

The noise sampler 22 is coupled to the matched filter 21. The noisesampler 22 typically is a gate for gating the output data signal fromthe matched filter 21, at particular times. The gating is the samplingof the digital output of the matched filter 21. The noise sampler 22samples, as illustrated in FIGS. 2 and 3, for each symbol sample at theplurality of chip times kT_(C), but not at the plurality of symbol timesnT_(S), the plurality of noise samples. The sampling of the plurality ofnoise samples may occur before, after, or a combination of before andafter, sampling of the corresponding symbol sample.

Timing for the noise sampler 22 and for the symbol sampler 23 may bederived from acquisition and tracking circuits 31 of the spread-spectrumreceiver. The acquisition and tracking circuits may derive timing from aheader portion of a packet signal, or from a separate synchronizationchannel. The acquisition and tracking circuits 31 generate timing whichcontrols a chip clock 32 for the desired spread-spectrum signal to bereceived. The timing circuit 33, based on timing from the chip clock 32,generates appropriate timing signals for triggering sampling of noisesampler 22 and symbol sampler 23.

The relative-signal-level means is embodied as relative-signal-leveldetector, which includes a delay device 41, a combiner 42, and acomparator 43. The delay device 41 is coupled to the symbol sampler 23.The delay device 41 delays an n-bit symbol sample, one symbol timeT_(S), thereby generating a delayed-symbol sample.

The combiner 42 is coupled to the delay device 41 and to the symbolsampler 23. The combiner 42 subtracts the delayed-symbol sample from thesymbol sample, thereby generating a relative-signal-level sample.

The comparator 43 is coupled to the combiner 42. The comparator 43 has athreshold input with a threshold, typically a voltage level. Thecomparator 43 compares the relative-signal-level sample to thethreshold, thereby generating each decoded-symbol sample of theplurality of decoded-symbol samples.

While the invention broadly applies to n-bit symbol samples, where n isthe number of bits per symbol, when the symbol samples are binary digitsor bits, then the relative-signal-level means might be embodied as adifferential decoder. Differential decoders are well known in the art.

The estimate means may be embodied as an estimator 44, such as aregister or memory circuit, for storing and averaging the plurality ofnoise samples. The estimate means may include a low pass filter, or analgorithm for computing or determining an average. The algorithm may be,by way of example, root means square averaging, means square averaging,straight averaging, weighted averaging, or determining a median value.

The erasure-detection means may be embodied as an erasure detector 45.The erasure detector 45 is coupled to the symbol sampler 23, theestimator 44 and the erasure decoder 28. The erasure detector 45, usinga particular symbol sample, from the symbol sampler 23, and acorresponding noise estimate from the estimator 44, generates an erasuresignal. Typically, the erasure detector 45 compares the symbol sample tothe noise estimate, and if the comparison failed to meet a certaincriterion or crosses a threshold, then the erasure detector 45 generatesthe erasure signal to erasure decode the corresponding symbols.

FIG. 4 illustrates detection between a symbol=1 and a symbol=0, withouterasure decoding, by comparing the output of the matched filter 21 to athreshold. FIG. 5 illustrates detection between a symbol=1 and asymbol=0, with erasure decoding. With erasure decoding, there is anin-between region, where an error has a likelihood of occurring. Thecomparison of the symbol sample and the noise estimate might be from asignal-to-interference ratio (SIR) or energy ratio, and if the SIR forthe particular symbol sample and noise estimate failed to cross athreshold, then the erasure signal indicates to erasure decode theparticular symbol sample. The criterion also may be based on the energyof the symbol sample, and noise estimate, or from subtracting the noiseestimate from the symbol sample. Other algorithms or criteria may beused, based on the symbol sample and the noise estimate, to determine ifthe symbol sample were to be erasure decoded.

In use, a plurality of spread-spectrum signals arrive at the input tothe receiver. The matched filter 21 detects the desired spread-spectrumsignal from the plurality of spread-spectrum signals, by having animpulse response matched to the desired chip-sequence signal. At theoutput of the matched filter, the symbol sampler 23 samples at eachsymbol time, nT_(S), to generate a plurality of symbol samples. Thenoise sampler 22, for each symbol sample, samples at a plurality of chiptimes kT_(C), to generate a plurality of noise samples. The estimatoraverages or filters, for each symbol sample, the plurality of noisesamples, to generate a noise estimate.

The erasure detector 45, for each symbol sample, uses a noise estimateto generate an erasure signal. The erasure signal is fed to the erasureinput of the FEC decoder 28.

The relative-signal-level detector decodes the plurality of symbolsamples, to generate a relative-signal-level sample. The n-bitrelative-signal-level sample, or magnitude of the relative-signal-levelsample, is fed to the data input of the FEC decoder 28. If the erasuresignal were present to erase the symbol sample, then the symbol sampleis erased at the FEC decoder 28 input.

The invention includes a method for improving a spread-spectrum receiverin a DS-CDMA system having a plurality of spread-spectrum signalsarriving at a base station from a plurality of remote stations. Eachspread-spectrum signal in the plurality of spread-spectrum signals hasrelative-signal-level encoded-symbol samples and a chip-sequence signallasting a symbol time. The chip-sequence signal is different from otherchip-sequence signals in the plurality of chip signals used by otherspread-spectrum signals in the plurality of spread-spectrum signals. Thespread-spectrum receiver has a matched filter with an impulse responsematched to a desired chip-sequence signal in the plurality ofchip-sequence signals. The matched filter detects a desiredspread-spectrum signal in the plurality of spread-spectrum signalsarriving at the spread-spectrum receiver.

The method comprises the steps of sampling at a plurality of symboltimes nT_(S), a plurality of symbol samples; relative-signal-leveldecoding the plurality of symbol samples, thereby generating a pluralityof decoded-symbol samples; sampling, for each symbol sample, at aplurality of chip times kT_(C), but not at the plurality of symbol timesnT_(S), a plurality of noise samples; averaging the plurality of noisesamples; detecting from the symbol sample and from the plurality ofnoise samples, an erasure condition, and thereby generating an erasuresignal; and erasure decoding the plurality of decoded-symbol samplesusing the erasure signals. Erasure decoding is well known to thoseversed in the art.

It will be apparent to those skilled in the art that variousmodifications can be made to relative-signal-level data detection from aspread-spectrum signal using matched-filter obtained side information ofthe instant invention without departing from the scope or spirit of theinvention, and it is intended that the present invention covermodifications and variations of relative-signal-level data detectionfrom a spread-spectrum signal using matched-filter obtained sideinformation, provided they come within the scope of the appended claimsand their equivalents.

I claim:
 1. An improvement to a spread-spectrum receiver at a basestation in a direct-sequence code-division-multiple-access (DS-CDMA)system, having a plurality of spread-spectrum signals modulatingdifferentially-encoded symbols, with each spread-spectrum signal in theplurality of spread-spectrum signals having a chip-sequence signallasting a symbol time T_(S), and with each chip-sequence signaldifferent from other chip-sequence signals used by other spread-spectrumsignals in the plurality of spread-spectrum signals, with thespread-spectrum receiver including a matched filter having an impulseresponse matched to a desired chip-sequence signal in the plurality ofchip-sequence signals, for detecting a desired spread-spectrum signal inthe plurality of spread-spectrum signals arriving at the spread-spectrumreceiver, the improvement comprising: a symbol sampler, coupled to saidmatched filter, for sampling at a plurality of symbol times nT_(S),where n is an index to each symbol time, a plurality of symbol samples;a relative-signal-level decoder, coupled to said symbol sampler, forrelative-signal-level decoding the plurality of symbol samples, therebygenerating a plurality of decoded-symbol samples; a noise sampler,coupled to said matched filter, for sampling at any of before, after, ora combination of before and after each decoded-symbol sample, at aplurality of chip times kT_(C), where k is an index of each chip timeTc, but not at the plurality of symbol times nT_(S), a plurality ofnoise samples; an estimator, coupled to said noise sampler, forprocessing the plurality of noise samples to generate a noise estimate;an erasure detector, coupled to said estimator and to said symbolsampler, for detecting from a particular symbol sample corresponding intime to the particular decoded-symbol sample and the noise estimate, anerasure condition, thereby generating an erasure signal; and an erasuredecoder, having an erasure input coupled to said erasure detector and adata input coupled to said relative-signal-level decoder, responsive tothe erasure signal, for erasure decoding the data input.
 2. Animprovement to a spread-spectrum receiver in a direct-sequencecode-division-multiple-access (DS-CDMA) system having a plurality ofspread-spectrum signals modulating differentially-encoded data, witheach spread-spectrum signal in the plurality of spread-spectrum signalshaving a chip-sequence signal lasting a symbol time T_(S), and with eachchip-sequence signal different from other chip-sequence signals used byother spread-spectrum signals in the plurality of spread-spectrumsignals, with the spread-spectrum receiver including despreading meansfor detecting a desired spread-spectrum signal in the plurality ofspread-spectrum signals arriving at the spread-spectrum receiver, theimprovement comprising: sampler means, coupled to said matched filter,for sampling at a plurality of symbol times nT_(S), a plurality ofsymbol samples; relative-signal-level means, coupled to said samplermeans, for relative-signal-level decoding the plurality of symbolsamples, thereby generating a plurality of decoded-symbol samples; saidsampler means for sampling at any of before, after, or a combination ofbefore and after each symbol sample corresponding to the particulardecoded-symbol sample from a plurality of decoded-symbol samples, at aplurality of chip times kT_(C), where k is an index of each chip timeTc, but not at the plurality of symbol times nT_(S), a plurality ofnoise samples; estimate means, coupled to said sampler means, forprocessing the plurality of noise samples to generate a correspondingnoise estimate for each decoded-symbol sample; erasure-detection means,coupled to said estimate means, for detecting from the correspondingnoise estimate and a symbol sample corresponding to each decoded-symbolsample from the plurality of decoded-symbol samples, an erasurecondition, thereby generating a corresponding erasure signal; and anerasure decoder, having an erasure input coupled to saiderasure-detection means and a data input coupled to saidrelative-signal-level means, responsive to the erasure signal, forerasure decoding the plurality of decoded-symbol samples.
 3. Theimprovement as set forth in claim 2 with said sampler means including: anoise sampler, coupled to said matched filter, for sampling at theplurality of chip times kT_(C), but not at the plurality of symbol timesnT_(C), the plurality of noise samples; and a symbol sampler, coupled tosaid matched filter, for sampling at the plurality of symbol timesnT_(S), the plurality of symbol sample.
 4. The improvement as set forthin claim 2, with said estimate means including: a delay device, coupledto said sampler means, for delaying a symbol sample one symbol timeT_(S), thereby generating a delayed-symbol sample; a combiner, coupledto said delay device and to said symbol sampler, for subtracting thedelayed-symbol sample from the symbol sample, thereby generating arelative-signal-level sample; and a comparator, coupled to said combinerand having a threshold input with a threshold, for comparing therelative-signal-level sample to the threshold, thereby generating adecoded-symbol sample of the plurality of decoded-symbol samples.
 5. Theimprovement as set forth in claim 3, with said estimate means including:a delay device, coupled to said symbol sampler, for delaying a symbolsample one symbol time T_(S), thereby generating a delayed-symbolsample; a combiner, coupled to said delay device and to said symbolsampler, for subtracting the delayed-symbol sample from the symbolsample, thereby generating a relative-signal-level sample; and acomparator, coupled to said combiner and having a threshold input with athreshold, for comparing the relative-signal-level sample to thethreshold, thereby generating a decoded-symbol sample of the pluralityof decoded-symbol samples.
 6. The improvement as set forth in claim 2, 3or 4, with said estimate means including a register for storing theplurality of noise samples.
 7. A method for improving a spread-spectrumreceiver in a direct-sequence code-division-multiple-access (DS-CDMA)system having a plurality of spread-spectrum signals modulatingdifferentially-encoded data, with each spread-spectrum signal in theplurality of spread-spectrum signals having relative-signal-levelencoded symbol samples and a chip-sequence signal lasting a symbol timeT_(S), and with each chip-sequence signal different from otherchip-sequence signals used by other spread-spectrum signals in theplurality of spread-spectrum signals, with the spread-spectrum receiverincluding despreading means for detecting a desired spread-spectrumsignal in the plurality of spread-spectrum signals arriving at thespread-spectrum receiver, the improvement comprising the steps of:sampling, at a plurality of symbol times nT_(S), where n is an index toeach symbol time, a plurality of symbol samples; relative-signal-leveldecoding the plurality of symbol samples, thereby generating a pluralityof decoded-symbol samples; sampling at any of before, after, or acombination of before and after each symbol sample, at a plurality ofchip times kT_(C), where k is an index of each chip time Tc, but not atthe plurality of symbol times nT_(S), a plurality of noise samples;processing the plurality of noise samples to generate a correspondingnoise estimate for each decoded-symbol sample; detecting from thecorresponding noise estimate and symbol sample corresponding to eachdecoded-symbol sample from the plurality of decoded-symbol samples, anerasure condition, thereby generating a corresponding erasure signal;and erasure decoding, in response to the erasure signals, the input-datasymbols.